As one of the display devices, a head-up display (which may be referred to as a HUD in the following) has been developed. In the HUD, an image displayed on a display panel is projected onto a windshield via a mirror. The projected image is visually recognized as a virtual image ahead of the windshield by an observer. The display panel to be applied in this projection displays an image by selectively transmitting light from a light source located at the back of the display panel, for example.
In the HUDs available recently, displaying various images at different positions is desired. If the size of the display panel is simply increased, the size of the light source for illuminating this display panel is also increased, which may cause an amount of generation of heat in the light source to be increased. Also, due to entry of external light in a unit of an optical system, the temperature in the unit may be increased. In addition, luminance of a virtual image is low as a result of a reflectance of a projection plane (for example, the windshield) being low. Therefore, if the brightness of the light source is enhanced in order to improve the visibility of the virtual image, the amount of heat generation of the light source may be increased. Such generation of heat by the HUD causes the optical system in the unit to be deformed, or the temperature of the space in which the HUD is mounted to be increased. Accordingly, a demand for suppressing the heat generation as much as possible is to be met. Also, making this type of HUD as small as possible structurally is demanded.